For Nocturne/DeSu2 veterans, what does this mean in practice OOC?
There's no hard evidence that they're connected; but at the start of Nocturne, prior to the Conception, you walk past the (closed by men in black) Yoyogi Park -- which is where Hikawa's final pre-Conception demon attack took place, and where he disabled some sort of 'radio tower'.
In Devil Survivors 2 you see the same style of tower in multiple locations, where they're used to counteract Polaris' erosion of the world, and it's mentioned there are multiple smaller/satellite towers besides the three major ones. Yoyogi doesn't come up, but it's well inside the protected range in Tokyo.
Considering that Yoyogi is close to the hospital where Conception actually starts, and said hospital would have been within range to be affected by said 'radio tower' if it were still operational, in the previous version of this quest I decided it's likely that said tower was doing the same thing as Fumi's towers; and that the clash between the Cult of Gaea and <unnamed government forces> in Nocturne might well have been between Hikawa and JPs, though possibly by proxy.
Sadly then, in the Nocturne timeline, they didn't catch on that the hospital in question was suspicious and needed to be investigated.
In the DeSu2 timeline, Hikawa's attack either fails or else doesn't happen at all.
Note that these games are both time loops. The 'true ending' of Nocturne is "Kagutsuchi is destroyed", but that breaks
every world and initiates a multiversal apocalypse; it's implied that it only happened after a large number of retries. The more probable ending is the 'neutral' one, where the demi-fiend is human enough to recreate the world as it was, but-
without Hikawa's plot, presumably. The demi-fiend appears to go back to human in this case, and there's no indication he keeps his memories. It's in breach of what Kagutsuchi is trying to achieve, and it does so under protest; it's unlikely to do him any favours he didn't ask for.
Meanwhile, the True Ending of DeSu2 is simply "time resets, but without the conditions that caused DeSu2" -- although in its case it's more likely that Hibiki keeps his memories, since Polaris' whole thing is more or less a test; it's just looking for someone who can overcome it, it's not entirely clear why, and it doesn't seem to particularly care what sort of world the victor then asks for.
These rhyme: The world gets recreated in both cases, to the specifications of the one who wins against the ultimate mastermind of the respective scenarios. It's also worth noting, however, that--despite Polaris being on the surface far more alien than Kagutsuchi--Kagutsuchi was looking for a
complete reset, a Reason that recreates humanity in fundamentally different conditions, whereas all of Polaris' accepted "Reasons" are basically variations on the level of "Let's try communism instead of capitalism", say.
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The conclusion, I think, is obvious enough. I'm still not going to draw it for you.